Considering how Birch is said the be the first life on this earth it is immediately useful as its young leaves, in moderation, are quite nutritious as a first Spring food. It's sticky sap within its bark is quite good as a sweetener and as a food ingredients binder.

As with all natural remedies consumption of Birch
should be guided by a qualified herbalist or physician.
This information here is for information purposes only,
and certainly not for prescription.

best use ...  

Added to a bath as it eases skins disorders and is very calming.

qualities of Birch are said to be ...

anthelmintic - cleanses parasites and worms
antirheumatic - relieves rheumatism
astringent - constricts tissues, such as stop bleeding, external use
choloagogue - supports flow of bile through gall bladder and liver
diaphoretic - promotes sweating
diuretic - increases urination, useful when clearing toxins
laxative - relieve of constipation
sedative - calming
stimulant - enhances alertness

relationship to its myth word?  birth  

This is not an obvious one, but bathing in birch tea does give a very young smooth skin feel

some "traditional remedies" from lore ...

Birch leaf tea is good for soothing mouth sores, kidney and bladder irritations.

Make a strong tea of Birch leaves, twigs and bark and then added to bath water helps eczema, psoriasis, and other skin problems. This is very useful if you cannot get to the sea to gather seaweeds to do the same.

Native American healers have used a yellow fungus that grows on Birch tops to heal tumours and body eruptions. Due to this, there is ongoing research by cancer treatment researchers.

toxicity? ...  as all plants have a nourishing level and toxic level

People with aspirin allergies and taking a warfarin or coumadin medication must consult a physician before consuming any birch remedy internally

Otherwise, it seems moderation of consumption of Birch material has been safe with users but does need care. The Birch leaves are much safer to use internally than twigs, bark or root. Externally, all parts of the Birch tree appear to be safe for humans to use

If you consume Birch tea, start with the leaves and make a weak tea. See how you react and then next time try a slightly stronger brew. Eventually, with guidance, you will discover what is best for you, but the guiding help of a herbalist is really essential if you intend to consume birch.regularly.

Though Birch has been used by traditional herbalists to remove internal toxins and parasites, over consumption of Birch can reverse the remedy so continued self medication with Birch is strongly not advised.

to read about the crafting potential of Birch, please click here